For example, in such a projection type projector apparatus, an image with uniform and sufficient color rendering properties is required to be projected onto a rectangle screen. For this reason, a metal halide lamp that encloses mercury and metal halide, is used as a light source. Moreover, in recent years, more miniaturization and development of a point light source are advanced, so that a lamp having a very short distance between electrodes has been put in practical use. In such a situation, a high pressure discharge lamp with very high mercury vapor pressure, for example, 20 MPa (approximately 197 atmospheric pressure) or more, has been recently used, instead of such a metal halide lamp. This lamp is designed to reduce arc spread, and to further improve an optical output by increasing the mercury vapor pressure.
For example, a high pressure discharge lamp in which a pair of electrodes is arranged in an arc tube made of silica glass, so as to face each other at an interval of 2 mm or less, and in which mercury of 0.20 mg/mm3 or more, rare gas, and halogen in a range of 10-6 μmol/mm3 to 10-2 μmol/mm3, are enclosed in the arc tube (for example, refer to Japanese Patent Application Serial No. H02-148561). For example, Japanese Patent Application Serial No. 2001-312997 teaches this kind of discharge lamp and a lighting apparatus. Japanese Patent Application Serial No. 2000-131668 teaches a high pressure discharge lamp in which the inside arc tube mercury vapor pressure at time of steady lighting is 15 MPa-35 MPa, and a halogen substance in a range of 10-6 μmol/mm3 to 10-2 μmol/mm3 is enclosed in the arc tube. Also, a pair of electrodes is provided in the arc tube and a projection portion is formed around the center of the tips of the electrodes to prevent an arc jump phenomenon, which is when the position of an electric discharge arc generated between the electrodes is not stabilized and moves between the central part or the circumference part of the tips of the electrodes. Further, alternating current voltage is impressed between the pair of electrodes so as to turn on the lamp, by a lighting apparatus, which is made up of a DC/DC converter, a DC/AC inverter, and a high pressure voltage generating apparatus.
On the other hand, since projectors are getting miniaturized in recent years and are standard in homes, it is necessary to consider a screen that does not become too bright in relation to the environment's brightness and a projected image. A projector that has a modulated light function was invented in order to meet such demands (for example, Japanese Patent Application Serial No. 2000-131668). Here, in the modulated light function, the brightness of a high pressure discharge lamp is adjusted so that power consumption is reduced to a low level by lighting the lamp with electric power lower than the rated power. Hereinafter, lamp lighting with electric power lower than the rated power is referred to as “modulated light power lighting.” In general, existing high pressure discharge lamp lighting apparatuses are equipped with both “rated power lighting” and “modulated light power lighting.” In the present specification, the lighting including the “rated power lighting” and the “modulated light power lighting” is defined as steady lighting. Moreover, the “modulated light power lighting” is in general operated with 60-80% of electric power in the “rated power lighting.”
FIG. 17 shows an example of a waveform of current in case where a high pressure discharge lamp is lighted by a lighting apparatus, which has a modulated light function. As shown in FIG. 17, when a modulated light power lighting command signal becomes ON while the rated power lighting is carried out, the high pressure discharge lamp is lighted with electric power which is decreased to approximately 60 to 80% of that in the rated power lighting to the electrodes.
Also in recent years, for example, a projector with a function called an “AV mute” has been invented. This function temporarily halts projection when an image projection to a screen itself is not needed. Since the internal pressure of a discharge lamp having such a function is high immediately after the discharge lamp is turned off, whereby it cannot be re-lighted at that time, light to be projected on the screen is blocked by shuttering it mechanically or adjusting voltage applied to a liquid crystal panel. Lighting of a lamp in a state where an image is not projected on a screen intentionally, is referred to as “standby power lighting”.
The electric power for standby power lighting is desirably at the utmost low level. By lighting the lamp with low electric power that is at the utmost low level, heat generated from the lamp is remarkably reduced, so that it is possible to stop driving or to reduce the speed of a cooling fan, which is a principal cause of a noise from the projector, thereby lowering the noise from the projector to the utmost level. Furthermore, since the thermal load of the lamp can be reduced by lighting the lamp with low electric power, it is possible to substantially extend a life span of the lamp by combining the cases where image projection is required with the cases where projection is not required. The “thermal load of the lamp” means a thermal load to an arc tube and electrodes, and they can be reduced by lowering applied power.
Moreover, a contrast ratio is one of performances of such a projector. The contrast ratio means comparing the luminance in a white state (bright state) on a projected screen to that in a black state (dark state) on the projected screen. Since a high contrast image can be projected with a high contrast ratio is high, a high contrast ration is considered as the important a feature in a projector as brightness.
The technology (such as an iris function), in which a black state is created by, for example, providing the above-mentioned mechanical shutter function in order to make a contrast ratio high, is adopted. There is demand for a high pressure mercury lamp, in which, while predetermined brightness or more is required in an operation of rated power as mentioned above, the modulated light power lighting is available, and further the standby power lighting in which modulated light power is extremely is restricted, is available.